In general, an electrophotographic image forming apparatus scans light onto an image receptor charged with a predetermined electric potential to form an electrostatic latent image, develops the electrostatic latent image into a toner image, and then transfers and fuses the toner image on a printing medium, so as to print an image. The fusing unit is arranged on a printing medium path in the image forming apparatus to fuse the transferred image on the printing medium.
For example referring to FIG. 1, a conventional fusing unit may include a heating roller 5 in which a source of heat, e.g., a heat lamp 3, is installed and a pressing roller 7 which is elastically biased towards the heating roller 5 so as to oppose and is the heating roller 5 to form a fusing nip therewith. As a printing medium 1 on which a toner image 1a passes through the fusing nip formed between the heating roller 5 and the pressing roller 7, the toner image 1a is adhered to the printing medium 1 by heat and pressure, thereby completing the fusing process.
In a fusing unit of the above-described configuration, the time required for the warming up can be relatively long because of the large heat capacity of the heating roller 5, resulting in a relatively long time for completing and outputting the first printed image.
Referring to FIG. 2, another example of a conventional fusing unit may include a heating film or belt 11 inside which a heat source 13 is installed; a nip plate 15 and a pressing roller 17. The nip plate 15 rotatably supports the heating film 11, and opposes the pressing roller 17 with the heating film 11 being interposed therebetween. The pressing roller 17 may be elastically biased towards the heat film 11 by an elastic member 19. With such configuration shown in FIG. 2, a relatively shorter warm up time, and thus a shorter time taken to output the first printed image, may be possible when compared with the conventional fusing unit shown in FIG. 1.
To operate conventional fusing units of the configurations shown in FIGS. 1 and 2 initially from resting state or during a printing medium jam state, the driving motor (not shown) rotationally driving the fusing unit may be subject to an increased level of torque.
As an illustration, plotted in FIG. 3 is the change in the torque and the rotational speed over time starting from the initial driving stage of the driving motor of a fusing unit that is, for example, configured as shown in FIG. 1.
Referring to FIG. 3, the driving torque of the driving motor in the initial driving stage of the fusing unit is about 16 kgf·cm, which is about 3 times of the driving torque once the fusing unit reaches a operational state, which is about 5 kgf·cm. It can also be observed, in this example, that the stable driving state is reached after the elapse of about 30 seconds. The rotational speed remains substantially constant at about 140 rpm. Similar torque change can be observed with respect to a fusing unit of the configuration illustrated in FIG. 2.
The increased level of torque during the initial driving stage or in the printing medium jam state as described above in conventional fusing units, such as, for example, those shown in FIGS. 1 and 2 may overload to the driving motor, and may thus adversely impact the useful operational life of the motor.